Make PSPP able to read all the portable files I could find on the

web.
* por-file-reader.c (struct pfm_reader): New member `line_length'.
(error): Print file offset in hexadecimal.
(warning): New function.
(advance): Treat lines less than 80 bytes long as padded to 80
bytes with spaces.
(pfm_open_reader): Call read_documents if we find an "E" record.
(convert_format): Convert invalid formats to the default format
instead of aborting reading the file.
(read_variables): Rename duplicate variable names instead of
aborting reading the file.
(read_value_label): Allow string variables of different widths to
be assigned value labels in the same record.  Replace duplicate
value labels instead of aborting.
(read_documents): New function.

* por-file-writer.c (pfm_open_writer): Call write_documents if the
dictionary has documents.
(write_documents): New function.